i. Technical Field
The present invention relates to an improvement in methods and apparatus for the batch digesting of cellulosic material such as wood chips, and more particularly to a process and apparatus for conserving the sensible heat contained in black spent liquor at the end of a digestion process.
ii. Prior Art
In conventional batch processes for digesting wood chips, the digester is filled with chips and the digester is then charged with a cooking chemical which in a soda process comprises essentially a solution of sodium hydroxide, and in a kraft process, comprises such a solution with a further inclusion of sulfur compound. The digester is then sealed and, with steam, the temperature of the digester is brought up to cooking temperature at which it is maintained for a period of time. At the conclusion of the cook, a blow valve in the digester is opened, and the contents of the digester is discharged into a blow tank by virtue of the hot liquor therein flashing into steam and forcing the delignified pulp out of the digester.
Much of the heat energy acquired by the contents of the digester during the processing exits through the blow tank with exhaust vapors. To recover such heat energy, attempts have been made to pass such vapors through various forms of heat recovery systems. Many of these recovery systems have not been efficient and, to conserve energy costs, some pulp manufacturers have chosen to install continuous digestion processes. A continuous process is quite distinctive from a batch digestion process, but usually has a more efficient utilization of heat than is achieved by a conventional batch process. However, the cost of equipment needed in a continuous process is normally substantially greater than the cost of equipment required in a batch type process, and the characteristics of the pulp obtained may differ.
Various arrangements have been proposed utilizing batch type processes which effect an energy saving such as those proposed in my U.S. Pat. Nos. 4,578,149 and 4,601,787. In the modified batch processes, at the end of a cook, the digester is held under pressure, and displacement liquids are used to displace the hot cooking liquors under pressure and substantially at cooking temperatures. Two or three accumulators are used to store the displaced cool, hot, and warm liquors in the three accumulator systems. During subsequent digester fills, the liquors in the accumulators are pumped to the digester to displace air and to preheat and pretreat the chips. All liquor fills are done by displacement. In the previously known displacement techniques, the displacing fluid is pumped into the bottom of the digester and the displaced fluid flows out the top of the digester.
An object of the present invention is to provide an improved method and apparatus which utilizes the advantages of a batch type process and which effects an increase in thermal energy saving over the more conventional batch processes.
A further object of the invention is to provide an improved batch type digester cooking system which employs a displacement concept of emptying the black spent liquor at the end of the digestion process and which effects a saving in time for removing the liquor at the end of the process.
A still further object of the invention is to provide a process wherein batch type cooking is employed and the black liquor is removed at the end of the cooking process by adding a displacement liquid wherein intermixing of displacement liquid and hot black liquor is diminished in order to conserve the high temperature of the spent liquor.